DESCRIPTION: Aggregatibater (Actinobacillus) actinomycetemcomitans is the etiologic agent of localized aggressive periodontitis and other systemic infections including infective endocarditis. A. actinomycetemcomitans produces an RTX toxin, leukotoxin (LtxA), specific for human and primate blood cells. LtxA acts as an immunotoxin, helping the bacterium to evade the immune response. We recently discovered that LtxA can lyse human red blood cells (RBCs) and have shown that this new function may play an important role in pathogenesis. Virtually nothing is known about the interactions between LtxA and RBCs since we only recently made this observation. The proposed experiments are designed to characterize and define the molecular interactions between LtxA and RBCs. We propose to first identify the RBC receptor for LtxA and next to define the domains of LtxA that are involved in binding to RBCs. Identifying critical interactions between LtxA and host RBCs may lead to the design of therapeutic agents that block interaction and subsequent disease. PUBLIC HEALTH RELEVANCE: The oral and systemic pathogen, Aggregatibater (Actinobacillus) actinomycetemcomitans, produces a toxin (leukotoxin) that targets human blood cells. Very little is known about the molecular interactions between the toxin and red blood cells (RBCs). Our goals are to identify and characterize the RBC receptor and domains in leukotoxin that are important for the toxin to bind RBCs.